Chromium (VI)-based aluminum-ceramic coating compositions have been well known and considered for decades as the industry standard for forming coatings which are highly corrosion-resistant and heat-resistant. U.S. Pat. No. 3,248,251 to Allen over forty years ago recognized and described the ability of aluminum-ceramic chromate-based coatings to exhibit resistance to corrosion, heat and abrasion while retaining adhesion and flexibility. Such attributes have continued to make aluminum-ceramic coatings widely used in a variety of applications. Today, these coatings are relied upon by original equipment manufacturers (OEM's) in the aircraft and power generation industries for protection of various turbine components subject to high temperature and corrosive environments. Department of Defense (DoD) depot facilities also use aluminum-ceramic coatings as part of their manufacturing for critically needed items. Additionally, the automotive industry and various other industries routinely use aluminum-ceramic coatings as a high performance protective coating.
The conventional aluminum-ceramic coating typically consists of an acidic chromate-phosphate binder that is filled with metallic aluminum powder. Upon curing, the binder forms a matrix that is embedded with the aluminum particles. The matrix provides mechanical integrity to the coating, while the chromate passivated aluminum pigment network imparts efficient corrosion protection. Burnishing Al-filled coating by dry grit or glass bead-blasting compresses the coating to render it conductive, galvanically active and sacrificial (i.e., cathodically protective) to all steels. SermeTel W® is recognized in the industry as the performance standard for these types of coatings. Depending on the particular application and service conditions, the coating can be used alone to provide adequate corrosion protection, or alternatively, the coating can be utilized as a part of an overlay system, serving as a basecoat that is sealed with top coatings and/or sealers. By sealing porosity and voids in the basecoat, the top coating provides additional barrier protection, thereby extending the corrosion protective action of the whole overlay system, as well as enhancing its other functional properties (e.g., smoothness, hot water resistance, heat oxidation resistance, engine fluids resistance, etc.) and its visual appearance.
Inorganic binder based top coats for high temperature resistant coating systems, as known in the art, are customary formed by a chromate-phosphate binder solution similar to that of the basecoat, but containing no metal particles. The topcoat composition may further contain metal oxide pigments that are chemically inert and heat resistant (such as Al2O3, TiO2, Cr2O3, multicomponent oxide spinels, etc.). Upon curing, the top coat forms a thin glass-ceramic type layer thus providing additional functional advantages.
In response to the identification of hexavalent chromium Cr(VI) as an environmentally hazardous material, various Cr-free aluminum ceramic base coats in combination with top coats have been investigated as potential environmentally benign replacement coating systems. Extensive efforts were placed into an approach which employed phosphoric acid based binders for both basecoat (filled with aluminum metal particles) and top coat compositions (pigment-free or filled with oxide pigments), as described in U.S. Pat. No. 5,968,240 to Myers et. al.; U.S. Pat. No. 6,224,657 to Myers et. al; U.S. Pat. No. 6,368,394 to Hughes et. al.; and U.S. Pat. No. 7,993,438 to Mosser et. al. For instance, one alternative Cr-free coating system of U.S. Pat. No. 7,993,438 considered is an aluminum ceramic basecoat layer having a phosphate-based binder. The coating when employed in conjunction with phosphoric-acid binder based Cr(VI)-free top coating such as in U.S. Pat. No. 6,224,657, provides application properties (e.g., thickness, roughness, galvanic activity) and performance (e.g., salt spray corrosion resistance, high temperature heat oxidation resistance, erosion resistance, mechanical properties) that meets OEM Specifications (although still not fully equal to the benchmark coating systems with SermeTel W® basecoat). However, as a stand-alone basecoat, the coatings developed red rust in the scribe and the field when subject to the Salt Spray test per ASTM B117 testing of up to 1000 hrs. Another drawback of this approach stems from a significant interaction of aluminum particles with the phosphate binder of the prior art in a water-based slurry in the absence of Cr(VI) species that have a passivating effect on aluminum metal. As a result of this adverse interaction of the aluminum particles with the phosphate binder, the basecoat slurry cannot be maintained as a “one-part” composition, in which all of the constituents can be mixed together into a single formulation, without one or more of the constituents adversely affecting other constituents of the composition. Rather, the slurry must be maintained in storage as a two-part slurry, in which the aluminum powder is maintained separate from the aqueous binder, until the point of use when the binder and Al can be mixed. However, the pot life of the mixed slurry is only about 8 hours, beyond which a rapid deterioration of the mixture is observed, that manifests itself in agglomeration of Al particles leading to a significant increase in the particle size. Although some specific modifications to the aluminum ceramic coatings employing phosphate-based binders can improve the pot life to over 24 hrs, the slurries must undesirably remain a two-part slurry to avoid the adverse interaction of the aluminum particles with the phosphate binder. To overcome the above problems, Applicants' U.S. patent application Ser. No. 13/673,007 relates to novel basecoat compositions exhibiting superior corrosion and heat resistance and capable of replacing traditional chromate-containing coatings. Specifically, as disclosed in U.S. patent application Ser. No. 13/673,007, Applicants discovered that utilizing a chromium-free silicate-based binder that is lithium-doped potassium silicate in combination with an aluminum powder produces a ceramic coating exhibiting improved functional properties, in particular long-term resistance to corrosion and heat exposure while retaining adhesion and flexibility. The coating layer is continuous, dense and defect-free. The slurry can contain both the Al powder and the binder so that the starting material is a one-part composition in which all constituents are pre-mixed as a single formulation. The one-part composition remains sufficiently stable to exhibit long shelf-life.
In a first aspect U.S. patent application Ser. No. 13/673,007 is directed to an aqueous slurry composition for the production of a coating on a substrate is provided. The composition comprises an aqueous binder comprising a lithium-doped potassium silicate solution in water. The binder is characterized by an absence of chromium. The slurry further includes an aluminum or aluminum alloy powder, wherein the aluminum or aluminum alloy powder comprises a particle size distribution characterized in that the 50th percentile of the particle size distribution has a diameter of between about 4 to 7 microns and the 90th percentile of the particle size distribution has a diameter of less than or equal to about 11.5-15.5 microns. The aluminum or aluminum alloy powder and the binder are contained as a one-part composition, each of which is contained in a predetermined weight ratio.
In a second aspect, U.S. patent application Ser. No. 13/673,007 is directed to an aqueous slurry composition for the production of a coating on substrate. The composition comprises an aqueous binder comprising a lithium-doped potassium silicate solution in water. The binder is characterized by an absence of chromium. The slurry further includes an aluminum or aluminum alloy powder contained as a one-part composition. The aluminum or aluminum alloy powder content in the slurry is between about 30-50 weight percent (wt %) based on the total weight of the slurry and further comprises a particle size distribution characterized in that the 50th percentile of the particle size distribution has a diameter of between about 3.9 to 4.5 microns and the 90th percentile of the particle size distribution has a diameter of less than or equal to about 9.0 microns.
In a third aspect, U.S. patent application Ser. No. 13/673,007 is directed to an aqueous slurry composition for the production of a coating on a substrate. The composition comprises an aqueous binder comprising a lithium doped potassium silicate solution in water. The binder is characterized by an absence of chromium. The slurry further includes aluminum or aluminum alloy powder incorporated into the binder as a one-part composition. The lithium-doped potassium silicate comprises potassium and lithium in a ratio of about 3:1 to 20:1 by weight as K2O:Li2O, and the ratio of silicate to potassium is in a ratio of about 2:1 to 3:1 by weight as SiO2:K2O.
In a fourth aspect, U.S. patent application Ser. No. 13/673,007 discloses a coating composition for a substrate comprising a ceramic matrix not containing chromium. The ceramic matrix is formed by a silicate binder and a plurality of aluminum powder particles embedded within the matrix. The silicate binder is potassium silicate doped with lithium. The aluminum powder comprises a particle size distribution characterized in that the 50th percentile of the particle size distribution has a diameter of between about 4 to 7 microns, and the 90th percentile of the particle size distribution has a diameter of less than or equal to about 11.5-15.5 microns.
In a fifth aspect, U.S. patent application Ser. No. 13/673,007 discloses a coating composition for a substrate comprising a ceramic matrix not containing chromium. The ceramic matrix is formed by a silicate binder and a plurality of aluminum powder particles embedded within the matrix. The silicate binder is potassium silicate doped with lithium. The aluminum powder comprises a particle size distribution characterized in that the 50th percentile of the particle size distribution has a diameter of between about 3.9 to 4.5 microns, and the 90th percentile of the particle size distribution has a diameter of less than or equal to about 9.0 microns.
Although the above lithium-doped potassium silicate binder based basecoat, as described in the parent U.S. patent application Ser. No. 13/673,007, has sufficiently good functional performance when used as a stand-alone coating, it would be desirable to achieve additional enhancement of the functional properties of the chromium-free basecoats disclosed in the parent U.S. patent application Ser. No. 13/673,007, such as, by way of example, increased corrosion protection, water resistance, heat oxidation resistance, etc. by employing these basecoats in conjunction with protective top coatings.
Chromate-free lithium-doped potassium silicate binder based basecoats of the parent U.S. patent application Ser. No. 13/673,007 demonstrate significant advantages to the Cr-free basecoat compositions of the prior art, hence there is further need to develop top coating compositions that are compatible with these basecoats. This, in turn, will allow creation of chromate-free basecoat-top coat coating systems with improved functional properties, including corrosion, heat and oxidation resistance.